Capacitive sensor circuits are widely used in various consumer electronic products for realizing functionalities such as accelerometers, gyroscopes, microphones, or the like. In a typical capacitive sensor circuit, a pre-processing circuit (e.g., an amplifier or a buffer) is adapted and connected between a capacitive sensor and a load for amplifying or buffering a sensing signal of the capacitive sensor. The amplifier or the buffer needs to be biased in a normal operation voltage; however, a capacitance value of the capacitive sensor is normally is too low for the amplifier or the buffer.
A technique for solving the above-mentioned problem is to add an input resistor with high impedance to an input of the amplifier or the buffer. Nevertheless, the input resistor with high impedance normally occupies a large component area, which is intolerant in circuit design.
Another technique for solving the above-mentioned problem is to add a transistor pair coupled in a current mirror type arrangement to provide impedance transformations. However, in some instances, the current mirror not only has an undesired non-linear output but also brings even order distortion, which further produces rectifying effects that shift a direct current (DC) operating point of the amplifier or the buffer.
Accordingly, the present disclosure provides a capacitive sensor circuit to overcome the aforesaid problems.